Defibrillation pacing circuitry

Abstract

Electrical circuit componentry is switchable into a defibrillator circuit to deliver a constant pacing current to a patient. The circuitry may include a constant current source inserted in a leg of the defibrillator circuit or a resistor of selected value inserted between a high voltage source and the high side of a defibrillator circuit.

What is claimed is: 
     
       1. An implantable cardiac stimulus system comprising:
 implantable electrodes for the delivery of electrical stimulus to a patient; 
 operational circuitry including at least an H-bridge circuit comprising a high side and a low side, wherein the H-bridge circuit comprises first and second legs connected between the high side and the low side thereof, wherein the first leg of the H-bridge circuit comprises first and second current switching elements and the second leg of the H-bridge circuit comprises third and fourth current switching elements, the H-bridge comprising output nodes for coupling to patient tissue via the implantable electrodes, wherein a first output node is located on the first leg between the first and second current switching elements and a second output node is located on the second leg between the third and fourth current switching elements, the operational circuitry further comprising a constant current circuit connected to the low side of the H-bridge circuit and 
 a canister for housing at least the battery and operational circuitry; 
 wherein the first and fourth current switching elements define a first pair of current switching elements; 
 wherein the operational circuitry is configured to select the first pair of current switching elements to generate a stimulus of a first polarity for application via the first and second output nodes; and 
 wherein the constant current circuit comprises a bypass switch and a pacing resistor in parallel with one another and coupled to the low side of the H-bridge; 
 wherein the operational circuitry is configured to close the bypass switch to deliver a defibrillation output, and to open the bypass switch to force current from the low side of the H-bridge through the pacing resistor to deliver a pacing output and 
 wherein the constant current circuit is configured to control operation of the fourth current switching element by monitoring a voltage across the pacing resistor during a stimulus of the first polarity to force the stimulus to be a substantially constant current pacing stimulus. 
 
     
     
       2. The implantable system of  claim 1  wherein:
 in the first leg of the H-bridge, the first current switching element links the high side of the H-bridge to the first output node and the second current switching element links the low side of the H-bridge to the first output node; 
 in the second leg of the H-bridge, the third current switching element links the high side of the H-bridge to the second output node and the fourth current switching element links the low side of the H-bridge to the second output node; 
 the constant current circuit includes an amplifier for generating a control signal output to the fourth switching element. 
 
     
     
       3. The implantable system of  claim 2  wherein the operational circuitry is configured to generate the substantially constant current pacing therapy by applying a first voltage to the high side of the H-bridge, closing the first switch, and applying the control signal output to the fourth switch. 
     
     
       4. The implantable system of  claim 3  wherein the operational circuitry further comprises a therapy circuit with a high power capacitor for use in delivering either pacing or defibrillator therapy, such that the first voltage is received from the high power capacitor.